diacidic

Diacidic is a term used in chemistry to describe a molecule that can donate two protons or that contains two acidic functional groups. It is often used interchangeably with diprotic or dibasic, though diacidic emphasizes the presence of two acidic sites and the potential for two-step deprotonation. Diacidic compounds are typically polyprotic acids, meaning they can exist in multiple ionization states depending on the pH. Deprotonation generally occurs in two steps, producing a monoanionic form after the first proton loss and a dianionic form after the second, with each step described by a separate dissociation constant.

Common diacids include dicarboxylic acids such as oxalic acid (HOOC-COOH), malonic acid (HOOC-CH2-COOH), and succinic acid (HOOC-CH2-CH2-COOH). Some amino acids, such as aspartic acid and glutamic acid, are diacidic in their side chains, capable of losing two protons under appropriate conditions. Inorganic examples include carbonic acid (H2CO3) and sulfurous acid (H2SO3), both diprotic.

In solution, diacids influence buffering capacity, metal complexation, and the properties of polymers and biomolecules. They play roles in analytical chemistry, biochemistry, and materials science, where their two removable protons and dual carboxylate groups can drive pH-dependent speciation, coordination behavior, and reactivity.

See also: dibasic, diprotic acid, dicarboxylic acid, polyprotic acid.